Tip and tail molds cracking / splitting

[ben_mtl]

As I need to buy some plywood I'll just get a sheet of 5/8" and replace all my horizontal flat MDF with plywood.. That should be a direct replacement process, no shimming needed (let's hope so !)
I believe the MDF sheets under the permanent mold might in a better shape than the top one as they're way less affected by temperature (my shop temperature is pretty steady, and even when pressing the high temps have to go through all the mold before getting there...) I'll see that tonight !


Regarding the flat part between the tip molds, you say it's gonna become concave after a few pressings. Do you mean the concavity will be the same as with MDF (=the material compresses making it thinner where most pressure is applied) or this sheet will remain the same thickness everywhere but it will bow because of the heat ?
If the concavity is caused by the material compressing, flipping it won't do anything, it the sheet is simply getting bowed as long as it's not too much I can live with it as the pressure will make it conform the "stay in place" mold (and eventually flip it once in a while as you said if it's too much of a PITA for loading the press..).

[Richuk]

Oh : ) I meant to say the flat part between the two tip molds is likely to become a little banana shaped - cambered would be a better way of putting it! If this happens, you will be able to flip it over (back to front) and it should roughly match the camber of your 'stay in place' section.

You're unlikely to see ski shaped impressions in the plywood. My AL sheets are thicker, but only partly for this reason.

[ben_mtl]

little update on the "stay-in-place" section, AKA camber mold.
I took the whole camber mold out of the press : as I said earlier the top surface is concave, that being said the bottom surface (resting on the press frame) looks like it's still good (= flat)
I removed the top MDF layer from the camber mold : to exterior (top) surface is concave, wich is not new, the bottom surface (resting on the vertically laminated ribs) is flat !
So the MDF basically compressed with the successive uses of the press.

Tomorrow I'll resurface the top of the vertical laminate as there are some glue residues, I'll also put a layer of 5/8 plywood under the camber mold in the press to compensate for the removed MDF.

That should solve part of my problem here until I decide it's time to renew my tip molds which was the original issue...

[NinetyFour]

I think one of the major players with your issue is definatley the heat. when testing my press I left my camber sheet underneath the hoses, and filled the top gap with wood. I had no cattrack in the press at the time, so it was just the round shape of the hoses pressing directly onto the MDF with a 3/4" cavity @ 60psi. I left the pressure on for 36 hours, and my MDF sheet still looks incredibly flat when compared to a square. No heat blankets.

MDF is 3/4" came from a 4x8" sheet at Home Depot. It weighed in at 100lbs if that makes any difference. Good luck with your issue!

[ben_mtl]

I agree with you : heat is the problem, but heat alone wouldn't to much arm.. the combo pressure + heat is what causes real trouble : MDF is manufactured from a sawdust&glue mix wich is compressed and heated... basically while pressing a ski with heat I'm repeating the manufacturing process, even if it's lower pressure and lower heat, no wonder it's getting slowly out of shape.

[NinetyFour]

Yeah I can totally agree with all of that. It still looks like some quality sticks are coming out of your press though! If it's working as is right now, I would stay with it, and look for some quality MDF before you re manufacture all of your ribs if you decided to do that. Either way, keep us updated! Nothing worse when a problem is solved and an answer is never published...

[falls]

Unfortunately I have just run into the same problem as you had ben.
The last 2 pairs of skis I have made have had a convex base. I pulled out my flat MDF pieces and they are definitely concave.
I agree it is the heat and pressure. I think mine have been worse because I have been making skinny (74mm waist) skis a lot lately. All the weight of the pressing is being centred over a narrower ski width (compared to 115mm waist) therefore the PSI being applied to the MDF is much higher than a fat ski as it distributes the force over a greater number of square inches.
I seem to have some concavity in my camber mold top surface, but also some in the MDF sheet below this (only a 10mm thick piece). The 1 inch plywood sheet below this seems to be pretty sound.

I am just hoping the shop can grind the last 2 pairs flat.

Anyone had their mold ribs cut out of plywood instead of MDF? Results?

[vinman]

If your molds are just compressed but not cracked, maybe resurface them with some drywall spackle?